Launching git gui can silently fail

If no `git-gui` is installed, launching the program fails without any error.

The problem was described in https://forum.qt.io/topic/141335/qt-creator-tools-git-git-tools-gitk-git-gui

The problem might be, that we call `git` with `QProcess::startDetached()` which succeeds, but `git` itself fails to run `git-gui`.

One solution might be use the same code as the `gitk` launcher, but I think that needs some cleanup before.

On the other hand, the problem seems Unix-only, so maybe it's enough to run `git-gui` on Unix and just keep the Windows code as is?

What do you think?